The present invention relates to a radio communication system, a radio relay device, and a recording medium. For example, the present invention can be applied to a radio multi-hop network.
In the case where each of radio communication devices (hereinafter, also referred to as “nodes”) cannot directly communicate with a management device that manages each of the nodes in a radio network system including the plurality of nodes and the management device, the radio network system may adopt a multi-hop network in which instead of a node that cannot perform direct communication, another node relays the communication. Such a multi-hop network enables a node at a long distance to communicate with a management device.
In the case where a node that newly participates in the multi-hop network (hereinafter, also referred to as a “participation node”) cannot directly communicate with the management device when the participation node participates in (joins) the network, the participation node selects a node (hereinafter, also referred to as a “parent node”) that relays a network participation request packet instead of the participation node, from among nodes that have already joined the network. In order to select such a parent node, the participation node broadcasts a packet (hereinafter, also referred to as an “adjacence node discovery packet) for discovering nodes (hereinafter, also referred to as “adjacent nodes”) around the participation node. An adjacent node that has received the packet returns a response packet to the participation node. At this time, if a transmission timing of the response packet from the adjacent node is the same as another adjacent node, packet collision happens, and the participation node cannot receive the response packet. Therefore, to avoid the collision, the adjacent node waits for random time after receiving the adjacent node discovery packet, and then transmits the response packet.
There are various ways to select an optimum parent node. For example, one of the ways is to select a node having the highest received signal strength (RSSI value) of a received packet. The participation node can decide the optimum parent node by collecting response packets from all adjacent nodes and selecting the parent node from among all the adjacence nodes. However, in order to collect the response packets from all the adjacence nodes, the participation node have to consider random waiting time of the response packets and wait for maximum waiting time that may be selected. Here, since a larger number of adjacent nodes results in higher probability of collision of the response packets, it is necessary to limit the collision probability to a certain level or lower. Therefore, the maximum value of random waiting time has to be set to be a little longer in view of the case of a large number of adjacent nodes.
As described above, since the participation node takes a lot of time to select the parent node, time to join the network is unfortunately prolonged.
Technologies described in JP 2009-302694A and JP 2011-523830T are technologies of methods for selecting an optimum parent node in a multi-hop network. According to the technologies described in JP 2009-302694A and JP 2011-523830T, a participation node receives information on an adjacent node (the number of current child nodes, the number of hops to the management device, round-trip time (RTT) to reach the management device, and the like) from the adjacent node, and selects an adjacent node that seems to be optimum as a parent node.
In addition, according to JP 2011-523830T, each node calculates a packet loss rate or signal-to-noise ratio (SN ratio) from a communication history with adjacent nodes, and dynamically changes the optimum parent node in accordance with such information even after participating in the network.